Steady, you're right, sportives do tend to plan their routes to take in as many hills as possible. It's great to not have to think about where you're going and just follow the arrows, but there's plenty of good routes locally to build up the miles.

If you can fit a couple 60-90 min sessions in during the week, you might find alternating relatively shorter (40-50) faster rides with longer (60-80) slower rides at the weekend suits you better than weekly long slow ones.

Doing at least one century ride will give you the confidence that you can do hte distance but if it stops you doing any other meaningful training for more than a few days it'd be counter-productive

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